Worldwide, commercial quantities of uranium hexafluoride (UF6) are stored and shipped in certified steel cylinders having 30-inch and 48-inch diameter. In order to remain in use, each of these UF6 cylinders must be washed internally and re-certified every five years. As part of the cylinder washing process, a small residue of UF6, called a heel, in the otherwise empty UF6 cylinder is first flushed with water from the cylinder's interior. Then the UF6 cylinder is subjected to a series of tests designed to verify its physical integrity. Successful completion of the physical testing re-certifies the UF6 cylinder for five additional years of service.
Cylinder washing and re-certification testing are usually performed as an ancillary effort at UF6 fabricators, enrichment facilities or conversion plants.